BACKGROUND. Hypoxia in solid tumors potentially stimulates angiogenesis by promoting vascular endothelial growth factor (VEGF) production and upregulating VEGF receptor expression. However, it is unknown whether hypoxia call modulate the effect of anti-angiogenic treatment on tumor-derived endothelium.METHODS. Human tumor-derived endothelial cells (HTDEC) were freshly isolated from surgically removed human colorectal tumors by collagenase/DNase digestion and Percol gradient sedimentation. Cell proliferation was assessed by measuring BrdU incorporation, and capillary tube formation was measured using Matrigel, Cell apoptosis was assessed by flow cytometry and ELISA, and Bcl-2 expression was detected by Western blot analysis.RESULTS. Under aerobic culture conditions (5% CO2 Plus 21% O-2) HTDEC expressed less Bcl-2 and were more susceptible to IFN-gamma-induced apoptosis with reductions in both cell proliferation and capillary,, tube formation, when compared with normal human macrovascular and microvascular EC. Following exposure of HTDEC to hypoxia (5% CO2 plus 2% O-2), IFN-gamma-induced cell apoptosis, and antiangiogenic activity (Le. an inhibition in cell proliferation and capillary tube formation) in HTDEC were markedly attenuated. This finding correlated with hypoxia-induced upregulation of Bcl-2 expression in HTDEC.CONCLUSIONS. These results indicate that hypoxia can protect HTDEC against IFN-gamma-mediated cell death and antiangiogenic activity, and suggest that improvement of tumor oxygenation may potentiate the efficacy of anti-cancer therapies specifically targeting the inhibition of tumor angiogenesis. (C) 2002 American Cancer Society.